A wide film is usually used for a covering material for agricultural use. A film to be industrially produced, such as a polyvinyl chloride film for agricultural use (hereinafter referred to as an agricultural PVC film) has a width of from 1 to 4 m, and films are bonded to one another to obtain a wider film which is used for a covering material for agricultural use. As a bonding method, it is common to employ a bonding method in which edges of two sheets of films are overlapped, and the overlapping portion is welded by press bonding under heating.
However, if fluororesin films are bonded by such a conventional bonding method, the entire area of the overlapping portion is press-bonded under heating at a temperature of at least the melting point of the fluororesin by a heat press bonding machine, whereby the strength of the welded portion in contact with the edge portion of the heater of the heat press bonding machine, tends to be low, and there will be a problem that the tear strength at the welded portion and its peripheral portion tends to be low. To solve such a problem, a bonding method may sometimes be employed wherein welding is carried out by press bonding under heating at a temperature lower than the melting point of the fluororesin. However, such a method has a problem that if the temperature is low, the overlapping portion may not sufficiently be welded, and consequently, the welded portion of the film is likely to undergo peeling.
Also in a case where such a fluororesin film is used as a material for a film structure, there will be a problem that the welded portion of the film is likely to undergo peeling, as mentioned above.
It is an object of the present invention to provide a method of bonding fluororesin films, whereby the welded portion is excellent in tear strength, has high welded strength and is not susceptible to peeling.
Another object of the present invention is to provide a wide fluororesin film having a width of from 1 to 150 m, having at least two sheets of fluororesin films bonded by repeating the above bonding method at least once, and a covering material for agricultural use, made of such a wide fluororesin film.